Lightguide fiber is drawn from a solid glass cylinder, or preform. The preform, having a central core surrounded by a cladding material, may be fabricated by the modified chemical vapor deposition (MCVD) process described in U.S. Pat. No. 4,217,027 which issued on Aug. 12, 1980 and which is incorporated herein by reference. Layers of doped silica are built-up on the inside of an elongated silica glass substrate tube by the reaction of glass precursor vapors, resulting in the formation of particles which deposit on and are fused to the inner wall of the tube. The composition of the precursor vapors is automatically controlled to give a step or graded index of refraction in the deposited glass layers which will form the core of the preform. When a fiber is drawn from the preform, the deposited silica glass becomes the lightguide fiber core and the silica glass tube becomes the fiber cladding.
In particular, vapors of material such as GeCl.sub.4, SiCl.sub.4, POCl.sub.3 or the like are entrained in a carrier gas such as oxygen and are directed, as a reactant vapor stream, into the interior of the glass tube which is rotated as an oxy-hydrogen torch repeatedly traverses its length. As the vapor stream passes through the tube and encounters a heat zone adjacent the torch it reacts, creating oxides which deposit on the interior surface of the tube. After numerous traversals of the torch along the length of the tube to deposit said layers, the tube is then subjected to elevated temperatures (e.g., 1900.degree. to 2000.degree. C.) by the torch in several traversals to shrink the tube and in a final traversal the tube is collapsed, resulting in a solid cylindrical rod shaped preform.
During this process not all of the reaction products are deposited within the preform tube but are exhausted from the tube in a powdery form along with exiting carrier gas. Heretofore these undeposited reaction products (i.e., soot) have been conveyed from the preform tube through a reactant collection tube formed as an integral extension of the preform tube. Gases and reactants passing through the collection tube are directed into a gas scrubber.
During the vapor deposition process, however, which ordinarily lasts for several hours, some of the soot exhausted from the preform tube deposit on and accumulate within the collection tube. This accumulation forms a progressively increasing restriction to the flow of fluids and other reaction products later passed through the collection tube which, in turn, affects the pressure of the vapor stream within the preform tube itself. Small changes in pressure and flow pattern at the exit end of the preform tube can substantially affect the deposition process as it is imperative that the vapors be delivered through the preform tube at precisely controlled mass flow rates. Thus, this progressively increasing restriction and changes in flow pattern within the exhaust tube adversely affects the deposition process within the preform tube since it is unpredictable and uncontrolled.
It is well known to place a cylindrical silica scraper rod inside the reactant collection tube. During rotation of the collection tube the rod agitates and shakes loose some of the reaction products that have deposited and accumulated on the interior walls thereof, thereby enabling the reaction products to be withdrawn by the vapor stream into the scrubber. Additionally, the scraper rod may be periodically moved about in both axial and radial directions in order to further clean the inside of the collection tube by scraping the powdery product that may have accumulated at the throat area where the collection tube is joined to the preform tube as well as in the collection tube itself. Such a technique provides improved results, however, it has not prevented some reaction product from accumulating and deleteriously affecting the deposition process. Even when agitated the products tend to diffuse upstream to some degree creating anomalies in the deposited layers within the preform tube.
Accordingly, there is a need for methods and apparatus for exhausting reaction products from lightguide preform tubes during layer deposition therein while providing a substantially uniform flow pattern and constant pressure at the exhaust end of the preform tube.